Previous studies have indicated that Guaroa virus (GV), an arbovirus, enhanced the replication of Friend leukemia virus (FLV) in mice. To study further the interaction of GV and FLV, different levels of FLV activity were inoculated into mice. At a level of activity capable of producing limited splenic response, coinfection with GV resulted in a marked increase in spleen-foci activity, mean spleen weight, and amount of infectious virus recoverable from plasma and spleen of dually infected mice. Various oncogenic and nononcogenic viruses were also tested for their ability to enhance FLV infection.